Presumably a double pole switch, with one pole cutting the HT and one pole cutting the heater supply to the phono section.
A 240V mains switch should survive- everybody uses them. The current levels are so low that you can over-voltage them quite a bit and still get long life.

Alternatively, you could just switch a resistor into the heater circuit so that the phono valves run with a reduced heater voltage (about 6V say), which will allow them to idle at reduced temperature. Since you're only dealing with low voltage, the switch rating won't be a problem.